1. Field of the Invention
The present invention relates to an image processing technology for correcting an image blur generated in image data.
2. Description of Related Art
In an imaging apparatus for converting an electric signal obtained from an image pickup device such as CCD (Charge Coupled Device) into a digital signal to obtain image data, a technique is known in the art, the technology for correcting a blur (hereinafter referred to as an image blur) generated in the image data due to camera shake. For such an image stabilization technique, various techniques are suggested including a method for detecting an amount of the image blur and a method for correcting the image blur using a detected motion vector.
A general operation of an image processing apparatus for correcting the image blur of image data obtained by an imaging apparatus for capturing a still image is described hereinafter in detail. Firstly the image processing apparatus inputs image data of a plurality of frames consecutively captured in response to a shutter pressed by a user. Then the image processing apparatus compares the image data of the plurality of frames being obtained in order to calculate a direction and a magnitude of the image blur (hereinafter referred to as a motion vector). Lastly pixel positions are shifted in order to cancel out the calculated motion vector and the plurality of image data are superposed. This creates the image with reduced image blur.
For example an image processing apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2005-295302 extracts a pixel having a large difference in pixel value from an adjacent pixel as a feature pixel, and evaluates a pixel displacement of the feature pixel among the image data of the plurality of frames. Then the image processing apparatus shifts the image data of the plurality of frames so as to correct the displacement of the feature pixel.
Further, various methods of calculating the motion vector are suggested. For example the image processing apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2005-295302 calculates the motion vector by evaluating a motion of the feature pixel. There is another method of calculating the motion vector from a correlation of pixel values for each block (see Japanese Unexamined Patent Application Publication No. 8-163566 and Japanese Unexamined Patent Application Publication No. 7-177425, for example).
Another method for evaluating a characteristic of the image blur from the direction of the calculated motion vector has conventionally been suggested. An imaging apparatus disclosed in Japanese Unexamined Patent Application Publication No. 8-251473 evaluates the characteristic of the motion vector by comparing the motion vector calculated for the captured image data with a peripheral motion vector so as to evaluate the image blur characteristic is due to a moving subject crossing the captured image. The imaging apparatus determines not to correct the image blur in case the evaluation suggests the image blur characteristics is due to a moving subject crossing the captured image.
In correcting the image blur by superposing the image data of the plurality of frames, image data not suitable for correcting the image blur by the superposition may be included in the plurality of image data. One of the image data not suitable for the image stabilization by the superposition is image data in which a spatial frequency of the image data is too high or low as a whole, so that the motion vector cannot be detected due to the small number of pixels to be feature points. Another image data not suitable for the image stabilization is image data in which a part of the image data is moving while being captured and image data including camera shake of rotation direction. Correcting the image blur by superposing the plurality of image data that includes such image data would deteriorate image quality, creating unnatural image after correcting the image blur.
As described in the foregoing, in correcting the image blur by superposing the plurality of image data, it has now been discovered that the image data after correcting its image blur could be deteriorated in case the plurality of image data include image not suitable for the correction.